1. Field of the Invention
The present invention concerns a method of operation of a transmission system of the type having four driving wheels of a motor vehicle and a method for controlling a means of mechanically coupling a first axle to a second axle of a transmission system of the type having four driving wheels of a motor vehicle. The invention also concerns a system for controlling the state of the mechanical coupling means. It also concerns a transmission system incorporating such a control system. It furthermore concerns a vehicle incorporating such a control system or such a transmission system.
The invention applies to a vehicle equipped with a system of four guided driving wheels. The objective of this type of vehicle is to improve capability such as performance and safety. In particular, the invention aims to improve the operating reliability of a mechanical coupling means by preventing its deterioration in extreme conditions of utilization, and to improve the traction performance of the vehicle equipped with the transmission system.
2. Description of Related Art
A transmission of the type having four driving wheels is known, in which a mechanical means of coupling a front axle to a rear axle (coupler) can operate according to three states defining three operating modes of the transmission system.                a first transmission mode where only a (variable) portion of the maximum torque capacity originating from a torque source (the engine of the motor vehicle) and intended for the rear axle is effectively transmitted to the rear axle; this first mode called “AWD Auto” mode corresponding to a coupler said to be “guided”, that is to say that a system for controlling the coupler automatically manages its operation, in particular the amount of torque transmitted or transmittable,        a second transmission mode where the totality of the maximum torque capacity originating from the torque source and intended for the rear axle is effectively transmitted to the rear axle; this second mode called “4WD Lock” mode corresponding to a coupler said to be “closed”, that is to say that it is locked in a state defining a fixed value of maximum transmittable torque,        a third transmission mode where no torque (or negligible torque) originating from the torque source is transmitted to the rear axle; this third mode called “2WD” mode corresponding to a coupler said to be “open”, that is to say that the coupler is deactivated or that it transmits only a negligible amount of torque.        
Document US 2003/0981 93 describes a method for distributing torque between the front and rear axles of a vehicle depending on the difference between the speeds of rotation of the front and rear axles. This method allows vehicle performance to be improved by optimizing slip without defining any compromise between reliability and performance.
U.S. Pat. No. 5,247,443 describes a method for distributing torque between the front and rear axles of a vehicle depending on the difference between the speeds of rotation of the front and rear axles, and which changes the detection threshold on slip if the vehicle service brake has been actuated. This method allows vehicle performance to be improved by optimizing slip without defining any compromise between reliability and performance.
The aim of the invention is to provide a method for controlling a means of mechanically coupling axles enabling the aforementioned problems to be remedied and improving the control methods known from the prior art. In particular, the invention proposes a method for controlling mechanical coupling means enabling the operating reliability of a means of coupling the axles of a motor vehicle of the type having four driving wheels to be improved while retaining good obstacle clearance performance.